1. Field of the Invention
The present invention relates to a non-aqueous secondary battery and its control method, particularly to a non-aqueous secondary battery preferably used for a energy storage system and its control method.
2. Description of the Prior Art
A household distributed power-storage system for storing nighttime power and photovoltaic power and a energy storage system for an electric vehicle have been recently noticed from the viewpoints of effective use of energy for resource saving and global atmospheric problems. For example, Japanese Unexamined Patent Publication No. 6-86463 discloses a total system constituted by combining a power supply from a power station, gas co-generation system, a fuel cell, and a storage battery as a system capable of supplying energy to energy consumers under an optimum condition. A secondary battery used for the above energy storage system must be a large scale battery having a large capacity unlike a small secondary battery for a portable devise having an energy capacity of 10 Wh or smaller. Therefore, the above energy storage system is normally used as a battery system constituted by stacking a plurality of secondary batteries in series and having a voltage of 50 to 400 V and in most cases, the system uses a lead-acid battery.
In the field of a small secondary battery for a portable devise, a nickel-hydrogen battery and a lithium secondary battery are developed as new batteries in order to correspond to the needs for a small size and a large capacity and therefore, a battery having a volume energy density of 180 Wh/l or more is marketed. Particularly, because a lithium ion battery has a possibility of a volume energy density exceeding 350 Wh/l and is superior to a lithium secondary battery using metal lithium as a negative electrode in reliabilities such as safety and cycle characteristic, the market of the battery has been remarkably expanded.
Therefore, also in the field of a large scale battery for a energy storage system, development is energetically progressed by Lithium Battery Energy Storage Technology Research Association (LIBES) or the like by targeting a lithium ion battery as a prospective product of a high energy density battery.
The energy capacity of the large lithium ion battery approximately ranges between 100 Wh and 400 Wh and the volume energy density of the battery ranges between 200 and 300 Wh/l, which reaches the level of a small secondary battery for a portable devise. Typical shapes of the battery include a cylindrical shape having a diameter of 50 to 70 mm and a length of 250 to 450 mm and a prismatic shape such as an angular box shape or a boxed shape with rounded edges having a thickness of 35 to 50 mm.
Moreover, as a thin lithium secondary battery, the following are disclosed: a film battery using a film obtained by laminating a metal and a plastic for a thin case and having a thickness of 1 mm or less (Japanese Unexamined Patent Publication Nos. 1993-159757 and 1995-57788) and a small prismatic battery having a thickness of 2 to 15 mm (Japanese Unexamined Patent Publication Nos. 1996-195204, 1996-138727, and 1997-213286). Purposes of these lithium secondary batteries correspond to decrease of a portable devise in size and thickness. For example, a thin battery which has a thickness of several millimeters and an area of approx. JIS size A4 and which can be stored on the bottom of a portable personal computer is also disclosed (Japanese Unexamined Patent Publication No. 1993-283105). However, the thin battery has an energy capacity of 10 Wh or smaller that is too small as a secondary battery for a energy storage system.
Japanese Unexamined Patent Publication Nos. 1982-208079 and 1988-24555 propose the use of graphite as a negative-electrode material for a lithium secondary battery which is superior in flexibility and on which mossy lithium is not deposited even in the case of repetition of a charge-discharge cycle. Because graphite has a special layer structure and a property of forming an inter-calation compound, it is practically used as an electrode material for a secondary battery using the property. Moreover, various types of carbon having a low crystallinity such as carbon having a disordered layer obtained by thermally decomposing hydrocarbon in a gaseous phase and a selective orientation property are disclosed in Japanese Unexamined Patent Publication No. 1988-24555 as materials in each of which an electrolyte is not easily decomposed.
These negative-electrode materials have advantages and disadvantages. When using carbon having a high crystallinity such as graphite as a negative-electrode material, it is theoretically known that a change of potentials due to discharge of lithium ions is decreased and a capacity to be used for a battery increases. However, when the crystallinity of the carbon is increased, the charging rate is lowered probably due to decomposition of an electrolyte, and the carbon is broken due to expansion/contraction of the plane interval of crystal caused by repetition of charge and discharge.
Moreover, when using carbon having a low crystallinity as a negative-electrode material, a change of potentials due to discharge of lithium ions increases and thereby, a capacity usable for a battery decreases, and thus, it is difficult to manufacture a battery having a large capacity.
Japanese Unexamined Patent Publication No. 1992-368778 shows that it is possible to prevent carbon from being broken by forming a double structure in which a carbon particle having a high crystallinity is covered with carbon having a low crystallinity. When using carbon prepared by the above method as an active material, it is theoretically possible to obtain an electrode superior in potential smoothness and having a large capacity by preventing decomposition of an electrolyte. When attempting formation of a practical electrode by using the double-structure active-material particles, an electrode having a thickness of 50 to 500 μm for a cylindrical battery by applying an active material onto copper foil. However, the capacity per electrode volume was not increased because the electrode density was not easily raised. More specifically, it is difficult to raise the electrode density. If setting the electrode density to 1.20 g/cm3 or more through pressure compression, a high volume capacity of 400 mAh/cm3 or more of the negative electrode cannot be resultantly obtained because the double-structure active material particles are broken.
In the case of a large lithium secondary battery (energy capacity of 30 Wh or larger) for an energy storage system, a high energy density can be obtained. However, because the design of the battery is generally similar to the small battery for a portable devise, a cylindrical or prismatic battery is constituted which has a diameter or a thickness three times larger than those of a small battery for a portable devise. In this case, heat is easily stored in the battery due to Joule heat caused by the internal resistance of the battery in charging or discharging or internal heat of the battery due to change of the entropy of the active material due to insertion or detachment of lithium ions. Therefore, the difference between the temperature of the inner portion of the battery and the temperature nearby the surface of the battery increases and thereby, internal resistances differ. As a result, charge capacity or voltage is easily fluctuated. Moreover, because two or more batteries of this type are connected to make a battery module in use, capability of heat storage differs depending on a battery position in the assembled system and fluctuation of heat storage between batteries occurs, and it is difficult to accurately control the whole of the battery module. Furthermore, because heat radiation is likely to be insufficient under high-rate charge/discharge, the battery temperature rises and thereby the battery is brought under an undesirable state. Therefore, a problem is left in the viewpoint of deterioration of service life due to decomposition of an electrolyte, and lack of reliability, particularly safety, because of the possibility of thermal runaway of a battery.
To solve the above problem, in the case of a energy storage system for an electric vehicle, the following methods are disclosed: air cooling using a cooling fan, a cooling method using a Peltier element (Japanese Unexamined Patent Publication No. 1996-148189) and a method for packing a latent-heat storing material into a battery (Japanese Unexamined Patent Publication No. 1997-219213). However, these methods use external cooling and therefore do not provide essential solution to the problems.
Moreover, to obtain a high-capacity battery, it is desirable to set a utilization factor of graphite-based particles used for a negative electrode to a value as high as possible. However, when improving the utilization factor, electrodeposition of lithium metal on a negative electrode increases and heat produced due to a reaction of an electrolyte at approximately 150 degree in Celsius increases. Particularly, in the case of a large scale battery, a negative electrode having a higher capacity is requested in order to improve the energy density and safety of the battery.
Furthermore, a separator having a thickness of 0.02 to 0.05 mm referred to as a micro-porous film made of polypropylene or polyethylene used for a commercially available lithium-ion secondary battery is a typical separator for the above lithium battery and it is locally attempted to use non-woven fabric of the above material for a separator.
In the case of a flat battery, the front and back surface areas of the battery increase as the thickness of the battery decreases, and holding force to be incurred on the surfaces of the electrodes in the battery decreases. Particularly, in the case of a large lithium secondary battery (energy capacity of 30 Wh or larger) used for a energy storage system, the above phenomenon is remarcable. For example, in the case of a 100 Wh-class lithium ion battery having a thickness of 6 mm, the front and back surface areas of the battery reach a very large value of 600 cm2 (either side).
Therefore, when using the above separator for a flat battery having a small holding force for pressing the surface of the electrodes, a problem is left that cyclic deterioration is accelerated due to the repetition of charge and discharge.
Moreover, as internal structure of a general battery, positive and negative electrodes and a separator for separating the electrodes from each other are layered. In the case of a lithium ion battery, a positive electrode made of metal oxide such as LiCoO2, a negative electrode made of carbon, such as graphite, which can be doped and undoped with lithium, and a separator referred to as micro-porous film made of polypropylene, polyethylene or the like and having a thickness of 0.02 to 0.05 mm are different from each other in dimension. For example, in the case of positive and negative electrodes, the negative electrode is designed so that it is slightly larger than the positive electrode to prevent electrodeposition of lithium metal on the negative electrode and to prevent fluctuation of the products even if faced positive and negative electrodes are slightly shifted from each other when a battery is assembled. Moreover, the separator is designed so that it is larger than the positive and negative electrodes in order to prevent a short circuit.
In the case of a cylindrical battery, positioning of the positive and negative electrodes and separators different from each other in size can be easily contrived in the operation of a winder. However, when stacking electrodes in a prismatic or box-shaped battery, the positioning is difficult. Therefore, in such cases, layered electrodes are made by pressing electrodes wound into an ellipse configuration, or by layering electrodes after inserting them into a baggy separator. However, a stacking method having a high packing efficiency of layers is desired.
Particularly, in the case of a flat battery, when using the method of pressing wound electrodes, a short circuit occurs due to separation of an electrode active-material layer from a current collector at an electrode portion having a intensively pressed curvature. When using a baggy separator, sufficient pressure cannot be obtained because of a large electrode area. Therefore, a gap is easily formed between a separator and an electrode layer due to creases or the like of the separator, and the internal resistance of the battery easily increases. Moreover, the binding margin of the separator increases in size and the packing efficiency of the electrodes decreases, influencing the capacity design of the battery. In view of the above-described points, a stacking method realizing a high packing efficiency of electrodes is not found which is suitable for a large scale battery or a flat large scale battery, simplifies positioning of layers, and hardly causes a short circuit.
To control a secondary battery for a energy storage system, in the case of an aqueous secondary battery such as a lead-acid battery or nickel-cadmium battery or the like, a plurality of single cells are connected in series to constitute a module and a plurality of modules are connected in series to constitute an assembly of batteries, in many cases. In these cases, charge and discharge operations are generally controlled per modules. By measuring voltage, temperature, current, and resistance of a module, the charge and discharge states and the deterioration level of a battery are determined, and charge and discharge are controlled in accordance with the determined results, in many cases.
In the case of a lithium ion battery, even a commercially available small secondary battery is generally controlled on cell by cell basis in a serial module (a module formed by serial connection of two cells or more). This is because a lithium ion battery has a large weak point in overcharge and overdischarge. For example, the safety of a cell become unsecured only by an overcharge state of several tens of mV, and overcharge or overdischarge fatally deteriorates a cycle life.
As described in Japanese Unexamined Patent Publication Nos. 1996-182212 and 1997-28042, a lithium ion battery for a energy storage system is also controlled on cell by cell basis. The single-cell control is the most advanced art among the battery control methods currently disclosed and is partly introduced into aqueous batteries for an energy storage system.
In the case of a large secondary battery (energy capacity of 30 Wh or larger) for a energy storage system, the capacity, volume, and electrode area for each single cell are ten times or more as large as those of a small battery for a portable devise and the fluctuation of operational characteristics in a single cell, which is not a large problem for a small secondary battery, reaches a level which cannot be ignored. Particularly, in the case of a large lithium secondary battery, the fluctuation of operational characteristics in a single cell is large and greatly influences the safety and reliability similarly to the fluctuation of operational characteristics between single cells of a small lithium ion battery.
Specifically, there are many fluctuations to be considered in a single cell such as electrode deterioration, contact pressure applied to an electrode, and current intensity in a current collector in the single cell. In the case of the above cylindrical and prismatic batteries (batteries having thickness and diameter three times or more as large as those of small battery for a portable devise), heat is easily stored in the batteries because of Joule heat due to the internal resistance of the batteries during charge or discharge, or because of internally produced heat of the batteries due to entropy change of active materials caused by insertion and detachment of lithium ion. Therefore, the difference between the temperature inner portion of the battery and the temperature nearby the surface of the battery is large, and thus the internal resistance showing temperature dependency differs, and the charge capacity and voltage are likely to fluctuate in a single cell.
However, because the large lithium secondary battery art of this type is generally similar to a small lithium ion secondary battery, attempts on battery design and charge and discharge control considering the fluctuation in a single cell are not made yet. Such attempts are not applied to aqueous secondary batteries such as a lead-acid battery, nickel-cadmium secondary battery, nickel-hydrogen secondary battery, which are generally controlled per module.